Most industrially utilised plastics consist of polymers having flexible chains. In contrast, polymers having relatively rigid polymeric chains have proven far less useful in an industrial context, primarily due to processing difficulties associated with their low solubility and high melting temperature. This is despite the fact that rigid polymers can exhibit some very interesting properties.
For example, polyacetylene and associated derivatives thereof can exhibit electrical conductivity (when doped), chemical reactivity and complex forming ability. Another potential application is chiral separation. However, polyacetylene is completely intractable, insoluble and infusible, and has found few practical applications.
Thus there is a long felt need in the art for rigid chain polymers which possess some or all of the advantageous properties usually associated with this class of material, but which also possess desirable features, such as excellent tractability, which are, traditionally, associated with polymers having flexible backbones.
Liquid crystals are well known materials which are of enormous importance in a number of applications, such as visual display. Molecules which are disposed to form liquid crystalline phases usually adopt either rigid, elongate or disc shaped molecular structures. Molecular structures that form liquid crystalline phases are called mesogens.
Ringsdorf et al (Macromol. Chem., 179 (1978) 2541) have proposed that liquid crystals can be formed from a polymeric main chain having a mesogenic side group attached thereto via a flexible spacer (in order to decouple the motion of the main chain from the mesogenic side group). To date, attention has focused almost exclusively on the use of flexible polymeric main chains. Thus, the standard molecular architecture of a side-chain liquid crystalline polymer (SCLCP) is: flexible backbone+spacer+mesogenic group. Few rigid chain SCLCPs have been developed, and often these rigid chain SCLCPs are used as examples of the destructive role played by stiff polymeric backbones. In general, rigid polymeric backbones are regarded as defects which distort the packing arrangements of pendant mesogenic groups in SCLCPs. Thus, the received wisdom in the field teaches away from the use of rigid main polymeric chains in SCLCPs.
Tang et al (Macromolecules 30 (1997) 5620) polymerised, with difficulty, a phenylacetylene having a side chain comprising a spacer group and a mesogenic group (poly({4-[((n-((41-cyano-4-biphenylyl)oxy)alkyl)oxy)carbonyl]phenyl}acetylene)—shown below as Structure I).
Unfortunately, the polymer was not mesomorphic, i.e. did not exhibit liquid crystalline behaviour. It was believed that the nonmesomorphism of I is due to the high rigidity of the polyphenylacetylene backbone, a conclusion which teaches against the use of polyphenylacetylene backbones in SCLCPS.
Akagi and Shirakawa (Macromol. Symp., 104 (1996) 137) have polymerised a group of mesomorphic acetylene monomers which contain an ether functionality (such as HC≡C(CH2)3O-biph-C5H11, where biph is biphenyl) using a MoCl5—Ph4Sn catalyst. Tang et al (Macromolecules 31 (1998) 2419) polymerised a number of mesomorphic polyacetylenes, namely poly{n-[((4′-cyano-4-biphenylyl)oxy)carbonyl]-1-alkynes}.